Producing a list of media library items categorized under a certain taxonomy item

You can use a tax query for all “Reference Type” terms, which will retrieve all attachments that have at least one “Reference Type” term.

$rt_media = new WP_Query(
    array(
        'posts_per_page' => -1,
        'post_type' => 'attachment',
        'tax_query' => array(
            array(
                'taxonomy' => 'reference_type',
                'terms' => get_terms(
                    'reference_type',
                    array(
                        'hide_empty' => false,
                        'fields' => 'ids',
                    )
                )
            )
        )
    )
);

while ( $rt_media->have_posts() ) : $rt_media->the_post(); ?>

    <div class="item">
        Use any template tags you want to output the list!
    </div>

<?php endwhile ?>